Because of the drive toward smaller, faster and denser microelectronic systems, a number of different techniques for nanolithography have been investigated. In general, feature sizes greater than 200 nm can be routinely produced by photolithography techniques. Electron beam lithography is commonly used to access length scales below that of photolithography, i.e., 200 nm down to 30 nm. However, feature sizes less than 30 nm are not easily obtained by standard semiconductor lithography techniques.
Techniques currently used for the fabrication of small structures on planar substrates are broadly classified as serial (one feature is created at a time) or parallel (an entire pattern or structure is made in a single step). Photolithography, the most commonly used technique in the microelectronics industry, falls into both categories. In this process, a mask is first prepared by a serial technique, which contains a highly specific and detailed pattern. The pattern contained in the mask can then be transferred in parallel (in a single step) to a substrate (e.g., a silicon wafer), by a photographic exposure and developing process. The minimum feature sizes attainable are typically 200 nm at present, and are determined by the wavelength of light used in the exposure, and the chemical properties of the substrate.
Serial techniques include electron beam lithography, and more recently, lithography based on scanned probes (scanning tunnelling microscope, scanning force microscope, etc.) . These offer the advantage of feature sizes as small as 20 nm for electron beam processes. These techniques offer precise control of feature placement. However, they are extremely slow due to their serial nature, and are best applied either to the fabrication of masks for subsequent parallel exposure, or to the fabrication of small numbers of devices for research or specialty applications.
Several methods have also recently been proposed which use a printing or stamping process to transfer small features which have been fabricated initially by a high resolution serial technique.
In addition, U.S. Pat. No. 4,512,848 discloses a process wherein an intermediate transfer mask consisting of a polymer is used to copy a master pattern. The intermediate transfer mask is then separated from the master pattern and placed on the surface of a substrate so as to form a lithographic mask. The pattern derived from the lithographic mask is then transferred to a substrate by etching.
An alternative strategy for nanoscale patterning is to use a "naturally occurring" or "self-assembling" structure as a template for subsequent parallel fabrication. For example, Deckman and Dunsmuir used a spin coating technique to prepare close-packed monolayers or colloidal polystyrene spheres with diameters of typically 0.1-10 microns on solid substrates. U.S. Pat. No. 4,407,695 and U.S. Pat. No. 4,801,476. The pattern is then replicated by a variety of techniques, including evaporation through the interstices, ion milling of the spheres and/or the substrates, and related techniques. Clark and Douglas used highly ordered biologically membranes ("S-layers") as starting points for fabrication, and processed these by techniques such as evaporation at a glancing angle and ion milling (Douglas et al. Science 1992 257:642). Close packed bundles of cylindrical glass fibers, which could be repeatedly drawn and repacked to reduce the diameters and lattice constant have also been used (Tonucci et al. Science 1992 258:738). Block copolymer films have also been suggested for use as lithography masks wherein micelles of the copolymer which form on the surface of a water bath are subsequently picked up on a substrate. Contrast comes from variations in the initial film thickness as formed on the water surface.
In general, techniques of this nature have both advantages and disadvantages. The potential advantages are (1) production of extremely regular arrays of features, with uniform size and spacing, on length scales which are difficult to access by e-beam lithography, and (2) their production in an extremely rapid, parallel fashion, over essentially unlimited large areas. The disadvantage is chiefly the restriction to a very limited number of patterns.
Accordingly, methods are needed for efficient, uniform nanometer periodic pattern formation and transfer to substrates which are suitable for various technological applications.